Geolocation refers to the process of determining geographic location of a subject of the process. The concept of Geolocation includes both source Geolocation, i.e., Geolocation of a transmitter (such as a cellular telephone, communication radio, radar); and self-Geolocation, i.e., Geolocation of the subject by the subject itself.
Source Geolocation is a classical problem for identifying the location of a transmitter, often done using triangulation. Passive Geolocation is commonly used to locate accurately a transmitter using multiple receivers (or, more precisely, receiver antennas) in non-multipath environments. Several triangulation methods may be used to locate signal sources in line-of-sight (LoS) environments. Geolocation of transmitters may use multilateration (e.g., triangulation) approaches, often with Time of Arrival (ToA), angle-of-arrival (AoA), power-of-arrival (PoA), frequency of arrival (FoA), and/or Time Difference of Arrival (TDoA) estimation techniques. Angle-of-Arrival, PoA, and FoA metrics may require favorable operating conditions. If the signal start time is known, ToA can be converted to range estimation. Emission time, however, is generally not available in the case of geolocation of a non-cooperative (hostile) transmitter.
In TDoA measurements, the signal of the transmitter is received at multiple receivers with distance-dependent time delays. Correlation analysis provides a time delay of the transmit signal corresponding to the path length difference to receiver pairs. When the signal is received at two receivers at known locations and TDoA, the intersection of possible transmitter locations lies on one half of a two-sheeted hyperboloid. Adding a third receiver at a third known location provides a second TDoA measurement (i.e., a second hyperboloid) with the location of the transmitter at the intersection of these two hyperboloids in two dimensions. A fourth receiver may enable measurement of a third hyperboloid, resulting in a determination of the transmitter location in three dimensions. Time Difference of Arrival techniques can be quite accurate for passive location estimation of a transmitter, including non-cooperative transmitter, when three or more receivers are available.
Geolocation may use Channel Impulse Response (CIR) estimations. In non-multipath environments, the CIR exhibits a single peak corresponding to the direct LoS. In this case, TDoA may be employed, because time of arrival can be determined directly from the peak.
In a multipath (MP) environment, a transmitted signal may be subjected to multiple scattering, resulting in a linear combination of delayed, attenuated, and Doppler-shifted versions of the original transmitted signal detected by the receivers along different paths. It follows that the CIR will exhibit multiple peaks and consequential uncertainty regarding which peak is the LoS peak, if LoS is present at all. Therefore, with significant MP contributions and/or without Los, the CIR may have multiple peaks, and measuring time of arrival becomes more difficult. Accurate and robust location estimation is thus challenging in harsh MP environments, with their time-varying MP fading and co-channel interference.
Low probability of intercept (LPI) communication techniques, such as direct sequence spread spectrum and frequency hopping, operate at instantaneous or average power levels that may be lower than ambient noise power levels. Such communication techniques present difficult scenarios for geolocation of non-cooperative transmitters. Low Probability of Intercept techniques may not communicate reliably in Non-Line-of-Sight (NLoS) multipath environments without incorporating Multiple-Input-Multiple-Output (MIMO) technologies. Geolocating signal transmitters in NLoS environments is more difficult, but possible with MP scattering. Multiple-Input-Multiple-Output systems generally have to ensure adequate decorrelated paths through the multipath environment, which means that they may transmit signals in all directions, allowing hostile receivers not only to detect but also to locate them, e.g., by combining triangulation and TDoA techniques.
Geolocation of hidden transmitters in an MP environment may be possible with recently developed processing techniques. The reason for this is that when the source is an NLoS source, a triangulation, by the hostile observer employing an array may first be used to locate the primary scatterer locations (from the point of view of the observer), and then iteratively find other scatterers and the original transmitter. This process is described in U.S. Provisional Patent Application Ser. No. 61/586,675, entitled Geolocation, filed on Jan. 13, 2012; and in U.S. Provisional Patent Application Ser. No. 61/597,492, entitled Geolocation, filed on Feb. 10, 2012. Each of these provisional patent applications (which are commonly owned with the present application and which were filed in the name of one of the inventors herein) is hereby incorporated by reference in its entirety as if fully set forth herein, including text, figures, claims, tables, computer program listing appendices (if present). In sum, TDoA techniques make geolocation possible when there are three or more discrete scatterers in the field. Each of these scatterers can be treated as a virtual antenna to enable both direction and distance of the source to be determined uniquely.
There is a need for techniques to hide transmitter locations in multipath environments in real-time, to prevent hostile receivers from locating signal transmitters, without unduly disrupting communications between the transmitters and their intended receivers.